Texture / Material offset doesn't work

Hey,

I want to make a 2D Sprite animation, buy changing the offset of the material depending on the rotation. So I made a material with the shader transparent/cutout/diffuse and the texture is a png image which has every step inside to "roll" with offset

There is an empty gameobject without a renderer. In this empty game object is a 3D text with Mesh renderer and the font material. in the empty game object there is also a plane with a mesh renderer, which has the material I want to "scroll" with offset.

Here is the script on the empty gameobject (reduced to the problem):

 using UnityEngine;
 using System.Collections;
 
 public class Player : MonoBehaviour {
 
  public Renderer mat1;
  public float i;
 
  // Use this for initialization
  void Start () 
  {
  mat1 = transform.FindChild ("playerPlane").renderer;
  }
  
  // Update is called once per frame
  void Update () 
  {
  i++;
  if(Input.GetKey (KeyCode.W))
  {
  mat1.material.mainTextureOffset = new Vector2(i,20f);
  Debug.Log(mat1.material.GetTextureOffset("_MainTex") + " - " + i);
  }
  }
 }

the print function works correctly, but the image doesnt change, I see something on the edges like black pixels that appear and dissappear, maybe something is moving, but not the image itself.

Anyone an idea?